He's a stout little lich

[Pants]

Well, I don't own the book, but evil Dwarven Liches could be similar to the Iron Lich in the Monsternomicon. A perfect combination of technology and magic. This might be a little too steampunkish for your campaign and it doesn't really fit in with your original idea, but I just through it out there.

 

[Olgar Shiverstone]

How about a dwarven shadow-lich, who remains behind to advise the elders of the clan as the "power behind the throne", or as the ultimate protector of the clan? Eventually, the clan dies off (bad advice, eaten by shadow dragons, overrun by goblins/drow/duerger, whatever), leaving the shadowy advisor behind to prosecute his own personal war against the invaders ... along whith his host of shadow-dwarf minions.

Maybe extreme greed pushes the rare dwarf to attain an undead state to continue accumulating wealth, instead of protecting his clan.

I like the earth-elemental take, though. Maybe over time they get slower and slower, and ancient dwarves become sentient rock. As consciousness fades over millenia, the fused bodies of the ancient dwarves hardens into an even harder form ... eventually mined as adamantine.

 

[Malk]

Nice, adamantine ancestral weapons take on a deeper meaning at that point.

 

[Olgar Shiverstone]

Nice, adamantine ancestral weapons take on a deeper meaning at that point.

And explains why dwarves will go to any length to recover "my grandfather's adamantine waraxe" They don't want the secret getting out ... it's why they try and control all the sources of adamantine (or already control all of them). A bit more fun than the "star metal" myth -- unless dwarves are originally from space -- nah!